1 Mr. ShieldsRegents Chemistry U12 L05

2 Rate of Sol’n We talked about HOW ionic solids go into solution But what factors affect how QUICKLY they go Into solution. In other words, what can we say about what Things affect the RATE OF SOLUTION? It turns our there are 3 factors that can affect The rate of sol’n: 1) Surface Area 2) Temperature 3) Mechanical Stirring

3 Surface area Let’s look at each of these 3 factors that affect the rate of solution. The first factor is SURFACE AREA From experience we know that 5g of granulated Sugar will dissolve faster than a 5g lump of sugar But why? What exactly is going on when sugar goes into Solution?

4 Surface area Since both water and sugar are polar molecules … 1. Polar water aligns it’s positive chg with sugars negative charge & vice versa 2. The dipole-dipole forces of attraction in sugar are overcome by the forces of attraction of the water

5 Surface area 3. Molecules leave the clumps of sugar - water surrounds the molecule in the solvation (in this case hydration) process Water Molecules Sucrose

6 Surface area So what’s the physical difference between Granular Sugar and a single lump of sugar? - Granular sugar consists of many small particles - And because we have these small particles their surface area is much larger than the surface area of the single lump OK. So why does the increased surface area increase the rate of solution?

7 Surface area With greater surface area many more particles Of sugar are exposed to water molecules per Unit time Therefore, more solvent comes into contact With sugar molecules more quickly Since more water Can come into Contact with more Solute, the solute Goes into sol’n More quickly Surface area: 108 “Lump of sugar” 9 x SA= 24 x 6 = 144 “granular sugar” 3 2 2

8 Temperature Now let’s see how temperature can affect the rate of Solution… What happens to KE when temp. increases? Recall KE = ½ mv 2 As Temp inc the KE increases, therefore Molecular velocity increases.

9 Temperature If solvent molecules are Moving faster at HIGHER Temperature what’s happening at the surface of the solute In a given period of time? 1.More molecules will reach the surface of the solute during a given time AND 2. Solvated ions can leave the area faster allowing new solvent molecules to arrive at the remaining unsolvated solute surface

10 Temperature 3. Lastly remember solvation (hydration) requires: a)Breaking the Force of attraction between solute ions (endothermic) b) Breaking the Force of attraction between solvent Molecules (also endothermic). Higher Velocity solvent molecules can provide this necessary energy. RATE

11 Stirring Stirring or agitation is the last thing we will Discuss that affects the rate of dissolving. This is a process we do almost subconsciously. For instance, when we make a cup of instant Coffee or when we add sugar to ice tea. But why does stirring increase The rate of dissolving?

12 Stirring As solvent molecules remove solute ions from the Crystal lattice surface several things happen: 1)The concentration of solvated solute in the Immediate vicinity of the crystal surface increases 2) This increased concentration of solvated solute Impedes new solvent from reaching the surface Of the solute.

13 Stirring Stirring (or agitation) “sweeps away” the increased Concentration of solute from the vicinity of The surface of the undissolved solute And Stirring also makes “fresh” solvent available at the Solute surface Solvated solute is concentrated Over solute surface Solute surface Solvated solute Fresh solvent sweeps Away solvated solute Exposing fresh solute Fresh solvent